The invention relates to a fluidized bed material which consists of a particulate mineral material. The invention relates also to a method for producing a fluidized bed material, as well as to a method in a fluidized bed process.
In fluidized bed processes, fluidized bed material in the form of mineral particles is used in connection with a reaction, such as a combustion reaction, taking place in a fluidized bed reactor, or in connection with material processing. The fluidized bed material forms a solid fluidized phase in the reaction or in the material processing, and its aim is to be inert and to stabilize the reaction conditions, such as the process of combustion. In combustion reactions, a solid material reactive under combustion conditions is also often added, such as limestone particles to adsorb sulphur.
In fluidized bed combustion, natural sand is presently used as the inert fluidized bed material, due to its easy availability. Sand contains primarily the following minerals, the contents varying as follows: quartz (SiO2) 25-70%, plagioclase (NaAl Si3O8+Ca Al Si3O8) 20-50%, and potash feldspar (KAlSi3O8) 10-30%. The percentage contents of the above minerals vary to a great extent according to the locations and conditions of formation of the sand.
The fuels used in fluidized bed boilers produce alkaline ash. When mineral quartz (so-called free quartz) contained in natural sand reacts with the alkali metals in the ashes of the fuel, it produces a gummy substance which acts like an adhesive between the particles of the bed material. This adhesive substance impedes the fluidizing by causing agglomeration of particles, and in the worst case it may cause the development of a whole solid sinter deposit.
The mechanism of agglomeration described in the paragraph above is only one possibility. Another possibility with ash-rich and alkali-rich fuels is a situation, in which ash melt is produced in such large quantities that adhesion takes place for physical reasons. Thus, the chemical reactions taking place on the surface of a particle of the fluidized bed material have no significance. The mechanisms leading to bed agglomeration are complex and at present insufficiently known.
Gasification and combustion of biomass by means of a hot fluidized sand bed is known e.g. from U.S. Pat. No. 4,968,325. U.S. Pat. No. 4,159,682, in turn, discloses predrying of a wet organic material to be combusted. The predrying takes place by fluidization with hot sand supplied from a fluidized bed boiler.
Because of the risk of sintering of the fluidized bed material in fluidized bed boilers, preventive measures must be taken that must be considered already in the design of the boiler construction. Thus, e.g. U.S. Pat. No. 4,942,673 discloses a device for preventing sintering, installed in the intermediate storage receptacle of the fluidized bed boiler.
Another way of preventing sintering is to supply the fluidized bed boiler with additives, which are usually metal oxides or substances producing them by decomposition, for the purpose of raising the melting point of the ash. The supply of the additive requires knowledge on the ash and right proportioning of the additive.
There is no unambiguous definition for a difficult fuel. In view of controlling the bed, a fuel is made difficult by its ash content (quantity of ash) and the composition of the ash produced. In the ash of a fuel, problematic for bed sintering are potassium (K), sodium (Na), sulphur (S), chlorine (Cl), and silicon (Si). In fluidized bed combustion, difficult fuels in view of bed agglomeration include e.g. different agricultural waste (straws and other fractions of different grains, almond shells, marc from olive oil), plywood, various animal excrements.
It is an aim of the present invention to present a fluidized bed material which has a considerably smaller risk of sintering and which can be used in the combustion of a variety of fuels, including difficult fuels, wherein additives are not necessarily needed. For attaining this aim, the material is primarily characterized in what will be presented in claim 1.
According to the invention, natural sand which is normally used in the fluidized bed reactor, is replaced by a quarried rock or mineral whose maximum free quartz content is 10 wt-%, preferably very small ( less than 0.1%). The definition includes naturally also all rock types or minerals containing no quartz at all. In this context, quartz refers to mineral quartz.
According to an established definition, rock types are natural mineral accumulations of certain composition and structure in the bedrock. Rock types are composed of sometimes one but usually several minerals whose grains are more or less tight joined to each other. Rock types do not have clear boundaries, because the ratios and occurrences of different minerals can vary within one and the same rock type. The term mineral refers in this context to a rock which is obtained from a natural deposit, consists primarily (more than 90-95 vol-%) of one mineral and may have small contents of accessory minerals. Thus, the terms rock type and mineral are not concepts that exclude each other in this context. Also minerals occurring in pure form in natural deposits are included in the scope of the invention.
It is also an aim of the invention to present a method for producing a fluidized bed material. In accordance with the invention, the method is characterized in what will be presented in the characterizing part of the appended claim 7. The fluidized bed material is produced in a suitable process by comminuting a natural rock type or mineral whose quartz content is suitably low or which is devoid of quartz. Thus, it is possible to select a material suitable for the fluidized bed process from a variety of possible rock types or minerals, and to grind the raw material to a suitable particle size, from which a fraction of suitable size can be separated by sieving.
It is also an aim of the invention to present a new fluidized bed process. The fluidized bed process is characterized in what will be presented in the characterizing part of the appended claim 13.